Napsin A is differentially expressed in sclerosing hemangiomas of the lung

Sclerosing hemangiomas (SH) are lung tumors characterized by surface cuboidal cells and round stromal cells. The cell of origin remains controversial, though immunohistochemical and ultrastructural studies suggest primitive respiratory epithelium. Napsin A, a human aspartic proteinase found primarily in type II pneumocytes and alveolar macrophages, is emerging as a helpful immunohistochemical marker in characterizing the origin of lung neoplasms, and may be of use in evaluating SH.

To evaluate napsin A immunohistochemical staining in SH to further characterize the cell of origin.

Six cases of SH were stained for napsin A, as well as thyroid transcription factor 1 and cytokeratin in selected cases.

Surface and round cells were positive for thyroid transcription factor 1 in all cases stained with this marker. Cytokeratins were positive in surface cells in all cases stained with this marker; 2 cases had focal cytokeratin staining in round cells. Round cells had focal napsin A staining in 1 case (17%); surface cells were napsin positive in all cases.

The observation of thyroid transcription factor 1 positivity in both surface and round cells in all SH suggests primitive respiratory epithelium as the cell of origin of SH. Our napsin A findings support this, with positivity in surface cells of all tumors (100%), and focal round cell staining in only 1 (17%). In fact, surface cells may represent entrapped type II pneumocytes, which normally express napsin A in a granular cytoplasmic pattern, similar to surface cells. The coexpression of thyroid transcription factor 1 and napsin A also introduces a caveat in differentiating primary pulmonary adenocarcinomas from SH in small biopsy specimens.